refactor: delegate traversal to vim.treesitter._select

Drop nvim-treesitter dependency and manual parent-walking loop.
Neovim 0.12+ built-in _select handles node traversal, injection
language trees, and normalization. Wildfire becomes a thin surround-
awareness layer on top.

- Extract surround detection into surround.lua using TS node
  structure (anonymous children as delimiters)
- Add pluggable checkpoint protocol (single trail stack) for
  selection undo history
- Switch visual-mode mappings to function callbacks so _select
  can read v/. marks correctly
- Remove: nvim-treesitter requires, count variable, fallback
  traversal, print_selection, duplicated coord conversion

BREAKING CHANGE: requires Neovim 0.12+

Author: fecet

lua/wildfire/checkpoint.lua

@@ -0,0 +1,48 @@
+--- Pluggable checkpoint protocol for selection history.
+---
+---@class wildfire.Checkpoint
+---@field save fun(self, buf: integer, selection: TSNode|integer[])
+---@field restore fun(self, buf: integer): TSNode|integer[]|nil
+---@field reset fun(self, buf: integer)
+---@field selection fun(self, buf: integer): TSNode|integer[]|nil
+---@field has_state fun(self, buf: integer): boolean
+
+---@class wildfire.StackCheckpoint: wildfire.Checkpoint
+---@field private _trail table<integer, (TSNode|integer[])[]>
+local StackCheckpoint = {}
+StackCheckpoint.__index = StackCheckpoint
+
+---@return wildfire.StackCheckpoint
+function StackCheckpoint.new()
+    return setmetatable({ _trail = {} }, StackCheckpoint)
+end
+
+function StackCheckpoint:reset(buf)
+    self._trail[buf] = {}
+end
+
+function StackCheckpoint:save(buf, selection)
+    if not self._trail[buf] then
+        self._trail[buf] = {}
+    end
+    table.insert(self._trail[buf], selection)
+end
+
+function StackCheckpoint:restore(buf)
+    local trail = self._trail[buf]
+    if not trail or #trail < 2 then return nil end
+    table.remove(trail)
+    return trail[#trail]
+end
+
+function StackCheckpoint:selection(buf)
+    local trail = self._trail[buf]
+    return trail and trail[#trail]
+end
+
+function StackCheckpoint:has_state(buf)
+    local trail = self._trail[buf]
+    return trail ~= nil and #trail > 0
+end
+
+return { new = StackCheckpoint.new }